a) Field of the Invention
The present invention relates to a real image mode variable magnification finder optical system which is to be used with photographic cameras, video cameras, still video cameras and so on.
b) Description of the Prior Art
For a camera composed of an imaging optical system and a finder optical system which are generally separated from each other, it is necessary to change a magnification of the finder optical system for adequate display of a photographic range the moment that a focal length of the imaging optical system is changed. The magnification of the finder optical system is changed, for example, in the manners described below:
First, in case of a finder optical system disclosed by Japanese Patent Preliminary Publication No. Hei 1-255825 disposed on the object side of an objective lens system is an auxiliary afocal lens system so as to be removable from an effective optical path of the finder optical system, and the magnification of the finder optical system is changed by inserting and removing this auxiliary lens system into and out of the optical path depending on a selected imaging magnification.
Further, in a finder optical system disclosed by Japanese Patent Preliminary Publication No. Sho 62-7107, an objective lens system is composed of three lens components having negative, positive and positive refractive powers respectively in order from the object side and the magnification of the finder optical system is changed by moving the second lens component in a direction along an optical system.
In a finder optical system disclosed by Japanese Patent Application No. Hei 2-146793 which was proposed by the inventor, in contrast, two objective lens systems having magnifications different from each other are fixed in parallel with each other and an optical path of one objective lens systems coincides with that of the other objective lens system by using two reflecting members. The magnification of the finder optical system is changed by rotating one of the reflecting members so as to change the optical path of one of these objective lens systems.
In a finder optical system disclosed by Japanese Patent Preliminary Publication No. Hei 1-255825 wherein the auxiliary lens system is inserted and removed into and out of the effective optical path of the finder optical system, however, it is necessary to reserve a space for disposing the auxiliary lens system and an additional space for allowing the insertion and removal of the auxiliary lens system. Therefore, this finder optical system has a remarkably long total length and has a thickness which can hardly be reduced. This fact poses an important problem in view of a fact that the finder optical system of this type is adopted frequently for various types of small cameras such as compact cameras. Further, since the auxiliary lens system which is to be disposed on the object side of the objective lens system must be of an afocal lens system, this auxiliary lens system must comprise at least one positive lens element and a negative lens element, thereby delimiting reduction of number of parts to be used for composing the finder optical system.
In the case of the finder optical system disclosed by Japanese Patent Preliminary Publication No. Sho 62-7017, a total length of the finder optical system is not varied by changing the magnification thereof. However, the objective lens system is composed, in order from the object side, of the negative, positive and positive lens components, a distance as measured from the first lens component to a focal point of the objective lens system is longer than a focal length of the first lens component, thereby posing a problem that a total length of the objective lens system can not be shortened inherently.
In case of the finder optical system disclosed by Japanesee Patent Application No. Hei 2-146793 wherein the magnification of the finder optical system is changed by rotating the reflecting member while the two objective lens systems are kept fixed, in contrast, the finder optical system has a total length which cannot be varied by changing the magnification thereof and the objective lens systems can independently be composed compact at magnification levels thereof, whereby the finder optical system can prevent cameras from being enlarged or permits designing the cameras compact. However, this finder optical system has a defect that it requires an increased number of parts since the two reflecting members are used in a pair for changing the magnification of the finder optical system.